There are several different track access methods in conventional optical disk drives. Among these, there is a method known as the track count method according to which the number of pulses of a detected track crossing signal are counted and the optical head is moved to a desired track by moving means such as a linear motor, all the while detecting the current position of the optical head based on the number of tracks crossed.
The conventional track count method is described hereinbelow. As shown in FIG. 7(a), guiding grooves 1 are provided at specified intervals on the surface of an optical disk, tracks 2 being formed between the guiding grooves 1. ID sections 3 are provided on the tracks 2. Each of the ID sections 3 shows a track number and a sector number etc. The information in the ID sections 3 is predeterminedly recorded by means of uneven pits having a specified depth (shown by hatching; individual pits are not shown).
During track access, although a light spot 4 moves in a direction orthogonal to or substantially orthogonal to the tracks 2, normally, since the optical disk is rotating during track access, a line 5 connecting the areas where the light spot 4 is incident on the optical disk becomes diagonal with respect to the tracks 2. Here, the light spot 4 is regarded as moving from an inner circumference toward an outer circumference within the region A-B shown in the Figure, and from the outer circumference toward the inner circumference within the region B-C.
FIGS. 7(b) and 7(c) respectively show the transition of a tracking error signal 6 and the transition of a total signal 7 when the light spot 4 moves as described above. The tracking error signal 6 is at zero level in a central section of each of the tracks 2 and the total signal 7 is at the maximum level in the central section of each of the tracks 2. Further, the tracking error signal 6 and the total signal 7 are modulated by the uneven pits when the light spot 4 passes the ID sections 3. As a result, jagged waveforms 6a and 7a, which include high-frequency components, appear.
In the case where a light detector (not shown) divided into two regions is used, the tracking error signal 6 is a difference, and the total signal 7 a sum of output signals released from each light receiving section of the light detector.
An envelope signal 10 shown in FIG. 7(d) is achieved by entering the total signal 7 into an envelope circuit 8 shown in FIG. 8. As a result, a waveform 10a of the envelope signal 10 corresponding to the ID sections 3 becomes substantially smooth. The envelope circuit 8 comprises an operational amplifier 11, a diode 12 connected to a non-inverting input terminal of the operational amplifier 11, and a capacitor 13 and resistor 14 connected in parallel between the non-inverting input terminal and the ground.
FIG. 7(e) shows a binary tracking error signal 15 which is a binary version of the tracking error signal 6. FIG. 7(f) shows a land-groove discrimination signal 17 achieved by comparing the envelope signal 10 with a specified slice level 16 using a comparator, not shown, and then binarizing it. When the light spot 4 is on one of the guiding grooves 1 (grooves), the land-groove discrimination signal 17 falls to a low level; when the light spot 4 is on one of the tracks 2 (lands), the land-groove discrimination signal 17 rises to a high level.
FIG. 7(g) shows a directional signal 18 which is achieved by latching the level of the land-groove discrimination signal 17 with the rising time of the binary tracking error signal 15. When the light spot 4 moves from the inner circumference to the outer circumference, the directional signal 18 falls to a low level; when the light spot 4 moves from the outer circumference to the inner circumference, the directional signal 18 rises to a high level.
An edge detecting signal 20 shown in FIG. 7(h) is a pulse which is released for a specified time when the binary tracking error signal 15 begins to rise. The edge detecting signal 20 corresponds to the timing with which the light spot 4 crosses the guiding grooves 1 when it moves from the inner to the outer circumference, and to the timing with which the light spot 4 crosses the tracks 2 when it moves from the outer to the inner circumference.
An up-signal 21 shown in FIG. 7(i) and a down-signal 22 shown in FIG. 7(j) are signals that are respectively achieved according to the edge detecting signal 20 in response to the logic level of the directional signal 18. That is, when the directional signal 18 is at a low level, the up-signal 21 is produced from the edge detecting signal 20; when at a high level, the down-signal 22 is produced. The number of pulses of the up-signal 21 correspond to the number of tracks 2 the light spot 4 crosses from the inner circumference to the outer circumference, and the number of pulses of the down-signal 22 correspond to the number of tracks 2 the light spot 4 crosses while moving from the outer circumference to the inner circumference of the optical disk.
Consequently, the amount by which the optical head moves in the radial directions of the optical disk can be detected by counting the up-signal 21 and the down-signal 22 using an up-down counter, not shown.
However, in optical disk drives of the track count type, it may happen that the amount by which the optical head moves in the radial directions of the optical disk cannot be detected accurately. Such a case is discussed hereinbelow.
FIGS. 9(b)-(j) show the transition of each signal of an optical disk using a Composite Continuous Format. Those signals having the same numbers as in FIG. 7 are the same signals as in FIG. 7.
As shown in FIG. 9(a), in the optical disk having this format, an ODF (Offset Detection Flag) section 23 is provided by interrupting guiding grooves 1 at specified intervals toward the posterior side of ID sections 3. A brief explanation regarding the ODF section 23 follows.
In optical disk drives which use rewritable-type optical disks, in order to ensure the power of the light beam during recording, and so on, the push-pull method, relying on a one-beam system, is commonly used for detecting tracking error. However, in this push-pull method, if an inclination of the radial directions of the optical disk or an abnormality in the angle of incidence of the light path etc. exists, a DC offset appears in a tracking error signal. That is, a problem occurs in that even if the tracking error signal is "0", it may happen that the light beam is not positioned on the center of a track.
The ODF section 23 is provided in order to resolve this problem. That is, the ODF 23 is a mirror-surface section whereon grooves and pits do not exist; therefore, no light diffraction takes place. As a result, the tracking error signal corresponding to the ODF section 23 should be "0". When, however, the tracking error signal corresponding to the ODF section 23 is not "0", this shows that an inclination of the optical disk, or an abnormality in the angle of incidence of the light path of the light beam has occurred. Consequently, when tracking control is being carried out, if the tracking error signal at the time when the light beam passes the ODF section 23 is detected and is used to correct the DC offset, the problem of the shifting of the light spot 4 from the tracks can be resolved.
Here, when a light spot 4 passes the ODF section 23, a tracking error signal 6 (FIG. 9(b)) falls to zero level, having a constant waveform 6b. A total signal 7 (FIG. 9(c)) rises to high level, having a constant waveform 7b.
When a total signal 7 is entered into the envelope circuit 8 shown in FIG. 8, a high charge builds up in the capacitor 13. Consequently, even after the light spot 4 has passed the ODF section 23, quite a lot of charge remains for a while in the capacitor 13. As a result, an envelope signal 10 acquires an attenuated waveform 10b, as shown in FIG. 9(d).
A land-groove discrimination signal 17, shown in FIG. 9(f), is achieved by comparing the envelope signal 10 with a slice level 16 using a comparator, not shown. In the vicinity of the ODF section 23, the land-groove discrimination signal 17 has a constant waveform 17a even when the light spot 4 passes a position where the guiding grooves 1 would have been if the ODF section 23 were not there. In the case where the ODF section 23 does not exist, the land-groove discrimination signal 17 becomes low-level, as shown by a broken line.
As a result, a directional signal 18 too switches correctly to low level and high level in response to the direction of movement of the light spot 4 in the case where the ODF section 23 does not exist, as shown by a broken line. In the case where the ODF section 23 does exist, the directional signal 18 acquires a waveform 18a which switches to a high level when the light spot 4 is moving from the inner circumference to the outer circumference.
Consequently, where a pulse 21a of an up-signal 21 (FIG. 9(i)) should have been produced, a pulse 22a of a down-signal 22 is erroneously produced. This makes accurate position detection of the optical head using the up-down counter impossible. As a result, the optical head cannot be accurately moved to the desired track.